Hello Prakash,
OSGi world is usually about reusability and dependencies. To expose services in the Talend ESB (and the Apache Karaf infrastructure generally) the CXF and built in Jetty is leveraged so the bundle with business logic can be very lightweight (inside the ESB container). As far I recall the older Talend ESB versions could export a standalone applications bundling all libraries inside, but the bundles themselves were very large, heavy and having hardcoded dependencies.
So even Talend could export jobs as standalone apps, for the web services (exposed by the tRestRequest component) I'd advice you to run the ESB container. I understand it can be heavy to run it for a single service (especially when you already have some app servers running), but as the number of services grows and you use the ESB as ESB (for routing, security, transformation, etc) it will pay off..
Gabriel